Presently, there is no early marker for the effectiveness of radiation therapy and phototherapy in the treatment of cancer. The goal of the present proposal is to document cellular and metabolic changes that occur in tumors in response to two therapeutic modalities, radiation therapy and phototherapy, administered at subcurative and curative treatment levels. In vivo 31P Nuclear Magnetic Resonance (NMR) will be employed to monitor intracellular pH and relative concentrations of high energy phosphate metabolites in conjunction with PO2 and pH microelectrode measurements. All measurements will be correlated with biological endpoints. Studies will be performed on C3H mouse mammary carcinoma. Experimental determinations made both during and following therapy will define both acute and long term response to the treatment. Our long term objective is to establish in vivo 31P NMR spectroscopy as a method for non-invasively assessing tumor response to radiation therapy and phototherapy.